Ferrisilicate molecular sieve

ABSTRACT

Ferrisilicate molecular sieves of the ZSM-5 type, having SiO 2  /Fe 2  O 3  mole ratios ranging from 20 to 400, are prepared by adding a silica source and a quaternary ammonium salt in that order to an acedified solution of an iron (III) compound, crystallizing the resulting gel to form a ferrisilicate molecular sieve, and thermally treating the molecular sieve with nitrogen, air and/or steam at 300° to 700° C. Preferred thermal treatment comprises treating with nitrogen first, then with air or steam. Thermally treated molecular sieves contain iron both in and outside the crystal framework; most of the non-framework iron is dispersed as very finely divided iron oxides or internal surfaces. Molecular sieves are useful as catalysts in Fischer-Tropsch and other iron oxide-catalyzed reactions.

This a continuation of applicants' co-pending application Ser. No.07/020,369, filed Mar. 2, 1987, now U.S. Pat. No. 4,952,385, issued Aug.28, 1990.

TECHNICAL FIELD

This invention relates to crystalline ferrisilicate molecular sieves.More particularly, this invention relates to crystalline ferrisilicatemolecular sieves of the ZSM-5 type.

BACKGROUND ART

Molecular sieves are ordered, porous crystalline materials having adefinite three-dimensional crystal structure, within which there are alarge number of small cavities which are interconnected by a number ofstill smaller channels or pores. These cavities and pores in anyspecific molecular sieve material are of precisely uniform size. Sincethe pores are of such size as to accept for adsorption molecules whichare small enough to pass through the pores, while rejecting molecules oflarger size, the materials have come to be known as "molecular sieves"and are utilized in various ways which take advantage of this property.Molecular sieves may be used, for example, as catalysts, selectiveadsorbents, drying agents, ion exchange materials, and for otherpurposes. Aluminosilicate molecular sieves are frequently referred to aszeolites.

The synthetic crystalline aluminosilicate zeolites are the best knownmolecular sieves. These materials are characterized by a rigidthree-dimensional network of SiO₄ ⁻ and AlO₄ ⁻ tetrahedra, which arecross-linked through shared oxygen atoms. The electronegativity of thealuminum-containing tetrahedra is balanced by the inclusion in thecrystal of a cation, typically monovalent or divalent, such as an alkalimetal (e.g. sodium) or an alkaline earth metal (e.g. calcium). Themonovalent or divalent ion is typically at least partially exchangeableby conventional ion exchange techniques. The aluminum and silicon arenot exchangeable. Various aluminosilicate molecular sieves are known.One of these is ZSM-5, which is described, for example, in U.S. Pat. No.3,702,886 to Argauer et al.

Less well known are the ferrisilicate molecular sieves. One of these,ZSM-12, is described in published European Patent Application (EPA) No.0013630. Another is the crystalline silicate described in U.S. Pat. No.4,208,305 to Kouwenhoven et al. This latter material is of the ZSM-5type and, according to the patent, consists structurally of athree-dimensional network of SiO₄, FeO₄, and optionally AlO₄, GaO₄ andGeO₄ tetrahedra which are interlinked by oxygen atoms. The patentdiscloses a number of catalytic processes in which the molecular sievesmay be used. However, direct conversion of a carbon monoxide-hydrogenmixture to a hydrocarbon mixture (the Fischer-Tropsch synthesis) is notamong these reactions.

Iron-containing zeolites are also known. These may be prepared by (a)physical admixture of a zeolite and an iron component, (b) ion exchangeof Fe (III) into a zeolite, (c) adsorption of a volatile metal compoundin the zeolite cavities followed by thermal decomposition, and (d)impregnation of a zeolite with a solution of a ferric compound followedby thermal decomposition.

In catalysts where the iron component is physically mixed with thezeolite ZSM-5, an intimate mixture between the two components is verydifficult to obtain. Thus, all of the iron component is likely to be onthe outside of the pores of the molecular sieve making these catalystsleast selective for the Fischer-Tropsch reaction. Further, the formationof large metal oxide particles decreases the amount of surface availablefor reactions to take place.

Loss of crystallinity and thermal stability is reported for syntheticzeolites which are ion exchanged with ferric ions. The ion exchange ofFe (III) cations into the zeolite can give rise to a high dispersion ofthe iron component. However, ion exchanging of Fe (III) ions into thezeolite ZSM-5 has not been completely successful, due to the size of thehydrated iron complex and the high dispersion of monovalent exchangesites within the zeolites.

Iron (O) (i.e., metallic iron) species can be introduced into the poresof a zeolite by adsorption and subsequent decomposition of the ironcomplexes. The most common volatile metal compound that is used toprepare iron containing zeolites is iron pentacarbonyl, Fe(CO)₅. Thesize of the iron pentacarbonyl is just about ideal to be adsorbed byzeolite Y. The iron pentacarbonyl is first adsorbed by the zeolite andthen the carbon monoxide is driven off from the iron pentacarbonyl bythermolysis. This process of making iron containing zeolites has thedisadvantage that during the process of thermolysis, the adsorbed metalcompound tends to come out of the pores of the zeolite. Moreover, theiron pentacarbonyl is too large in size to enter the zeolite ZSM-5 andhence when used over zeolite ZSM-5 will have all of the iron presentoutside the pores of the zeolite ZSM-5.

DISCLOSURE OF THE INVENTION

This invention according to a first aspect provides novel crystallineferrisilicate molecular sieves of the ZSM-5 type. These molecular sieveshave an overall silica to ferric oxide (SiO₂ /Fe₂ O₃) mole ratio in therange of about 20 to 400. A first portion of the iron content is in thecrystal framework or lattice, and the remaining portion of the iron isoutside the crystal framework. This remaining portions constitutes fromabout 0 to about 80 percent by weight of the total iron content and isdispersed in the form of finely divided particles on the internal andexternal surfaces of the molecular sieve. At least about 30 percent,preferably at least about 50 percent, most preferably at least about 80percent, of the non-framework iron is dispersed on the internalsurfaces. Nearly all of the non-framework iron on the internal surfacesis in the form of iron oxide particles having a particle size less thanabout 5 Angstrom units, while iron on the external surfaces is in theform of iron oxide particles predominantly from about 5 to about 15Angstrom units.

This invention according to a second aspect provides a process forpreparing ferrisilicate molecular sieves of the ZSM-5 type. This processcomprises: (a) adding a silica source and one or more compounds selectedfrom the group consisting of primary, secondary and tertiary amines andquaternary ammonium compounds to an acidic aqueous solution of an ironcompound, and maintaining said solution in the acidic state until theaddition of said silica source is complete; (b) heating the mixtureobtained in step (a) at a temperature of about 100° to about 250° C.until molecular sieve crystals are obtained; and (c) thermally treatingthe molecular sieve crystals formed in step (b) at a temperature fromabout 300° to about 1000° C. The preferred thermal treatment comprisestreating the molecular sieve of step (b) in an inert atmosphere at about450° to about 800° C. for about 6 to about 16 hours, then in air atabout 400° to about 1000° C. for about 3 to about 8 hours, and thenoptionally with steam at about 250° to about 700° C. for about 0.5 toabout 36 hours.

This invention according to a third aspect provides a process forcarrying out chemical reactions using a ferrisilicate molecular sieve ofthe ZSM-5 type as described above. According to this process, a gaseousreactant or mixture thereof is contacted with the ferrisilicatemolecular sieve under reaction conditions. In particular, this processmay be a catalytic process in which a mixture of carbon monoxide andhydrogen is contacted with the molecular sieve at a temperature fromabout 250° to about 400° C. and at a pressure from about one to about 20atmospheres, whereby a reaction product comprising a hydrocarbon ormixture thereof is obtained. This hydrocarbon or mixture thereofcomprises a major portion of gasoline range (C₆ to C₁₀) hydrocarbonswhen the temperature is maintained from about 250° to about 350° C. andthe pressure is from about 10 to about 20 atmospheres.

BEST MODE FOR CARRYING OUT THE INVENTION

The ferrisilicate molecular sieves of the present invention have anoverall SiO₂ /Fe₂ O₃ mole ratio in the range of about 20 to about 400,preferably from about 30 to about 200, and exhibit ZSM-5 structure.These molecular sieves consist structurally of a three-dimensionalframework of SiO₄ ⁻ and FeO₂ ⁻ tetrahedra which are interlinked bycommon oxygen atoms.

Only a portion of the iron content of final product (i.e., thermallytreated) molecular sieves of this invention is in the framework.Framework iron is in the form of tetrahedra. Framework iron mayconstitute as little as 20 percent (by weight) of total iron; typicallyhowever, framework iron is from about 50 to 100 percent of the totaliron content. The remainder of the iron is outside the framework in theform of octahedra, and consists essentially of finely divided particlesof iron oxides dispersed on the internal and external surfaces of themolecular sieve. Nearly all of the particles on the internal surfacesare smaller than about 5 Angstroms (A) in size. Particles on theexternal surfaces are predominantly from 5 to 15A. Most of thenon-framework iron is dispersed on the internal surfaces, i.e. surfacesof the pores and the cavities (which for convenience will simply bereferred to as the pore surfaces) of the sieve. The thermally treatedmolecular sieves may range from off white to brown in color.Distribution of the iron content of the molecular sieve between theframework and non-framework sites may be shown by Mossbauer spectra.Thermally treated molecular sieves of this invention have a high degreeof thermal stability.

The electronegativity of framework iron is balanced by exchangeablecations, e.g. hydrogen, ammonium, alkali metal or alkaline earth metal,in the crystal structure. The ion exchange capacity of a productmolecular sieve furnishes a quantitative measure of the amount offramework (tetrahedral) iron present. Thermally treated molecular sievesas produced are in the hydrogen form; other exchangeable cations may beintroduced by conventional ion exchange techniques. The overall SiO₂/Fe₂ O₃ mole ratio of a thermally treated molecular sieve herein isbased on the total quantity (framework plus non-framework) iron present.

The ferrisilicate molecular sieves of this invention exhibit ZSM-5structure and may be regarded as analogs of the known crystallinealuminosilicate zeolite molecular sieves. Such molecular sieves aredescribed, for example, in U.S. Pat. No. 3,702,886 cited above. Oneindication of ZSM-5 structure is the presence of pores of a uniformdiameter of about 5.5 Angstroms. Another indication is an x-raydiffraction pattern which is similar to that of known ZSM-5 molecularsieves. The x-ray diffraction pattern of the molecular sieves of thisinvention is shown in Table I below.

                  TABLE I                                                         ______________________________________                                                               Inten-                                                 Number   2, Theta      sity    I/Io                                           ______________________________________                                         1        7.78         689     28                                              2        8.72         520     21                                              3       11.68         153     6                                               4       13.66         156     6                                               5       13.84         226     9                                               6       15.78         192     7                                               7       17.6          116     4                                               8       19.14         162     6                                               9       20.22         252     9                                              10       22.06         173     7                                              11       22.96         2144    88                                             12       23.14         2431    100                                            13       23.58         676     27                                             14       23.82         1105    45                                             15       24.28         780     32                                             16       25.64         130     5                                              17       26.5          180     7                                              18       26.84         240     9                                              19       29.14         212     8                                              20       29.78         236     9                                              ______________________________________                                    

As synthesized ferrisilicate molecular sieves of this invention arecrystals having a white to pale lemon yellow color, indicating that allor most (e.g., at least 90 percent) of the iron content is in theframework, and having the same mole ratio of silica to ferric oxide(i.e. from about 20 to about 400) that characterizes the final product.The percentage of iron in the framework is lower at SiO₂ /Fe₂ O₃ moleratios below about 50. The as synthesized ferrisilicate molecular sievesmay be represented on the water free basis by the following formula:

    aR.sub.2 O.Fe.sub.2 O.sub.3.bSiO.sub.2

where R is alkylammonium, dialkylammoniom, trialkylammonium ortetraalkylammonium; a is from about 1 to about 6; and b is from about 20to about 400. R is preferably tetraalkylammonium, and the alkyl groupsare lower alkyl groups, i.e., alkyl groups containing from one to about8 carbon atoms. A minor amount of R may be accounted for by an alkalimetal ion, e.g. sodium.

The x-ray diffraction pattern of the as synthesized molecular sieve issubstantially the same as that of the final product molecular sieve,i.e., as shown in Table I.

Preparation of the product ferrisilicate molecular sieves of thisinvention requires two operations, i.e. (1) preparation of the assynthesized ferrisilicate, and (2) thermal treatment of the assynthesized ferrisilicate in order to form the product ferrisilicatemolecular sieve.

The as synthesized ferrisilicate is preferably formed by adding a silicasource to an acidic solution of an iron (III)(i.e., ferric compound,adding to the resulting gel a primary amine, a secondary amine, andtertiary amine, or a quaternary ammonium salt and heating the resultingmixture (which is a gel), preferably in an autoclave under autogenousconditions at about 100° to about 250° C., until crystallization takesplace.

The acidified solution of a ferric [i.e., ions (III)] compound isobtained by dissolving an iron (III) compound, such as ferric nitrate,ferric chloride or ferric sulfate, in water and acidifying the resultingsolution with a strong mineral acid such as hydrochloric or sulfuricacid to pH not higher than about 5.

The silica source (or precursor) may be either an aqueous solution of analkaline metal silicate or an aqueous silica sol. Alkali metal silicatesolutions are ordinarily preferred, because these result in betterincorporation of the iron into the framework, while use of a silica solresults in a substantial amount of non-framework (octahedral) iron inthe as synthesized ferrisilicate. Representative alkaline metalsilicates are N-Brand silicate (PQ Corporation), which has the formulaNa₂ SiO₃.5H₂ O. Sodium metasilicate from other vendors can also be used.Other sodium silicates having different SiO₂ /Na₂ O mole ratios may alsobe used. Representative silica sols (less suitable as previouslyindicated) include "LUDOX" (E. I. Dupont Company) and "Cab-O-Sil" (CabotCorp.), both of which contain particles of high molecular weightpolymeric silica beads.

It is important to add the silica source to the iron (III) solution,rather than to add the iron solution to the silica source or to chargeboth simultaneously to a reaction vessel, because it is important tomaintain an acidic pH, preferably below about 5 throughout the additionof the silica source. If this is not done, iron (III) hydroxide willprecipitate and the desired incorporation of substantially all of theiron into the framework of the as synthesized ferrisilicate gel, and thedesired distribution and particle size characteristics of the iron inthe final product molecular sieve, will not be obtained.

The amine or quaternary ammonium salt is preferably added after theaddition of the silica source is complete. The amines are primary,secondary or tertiary alkyl amines in which the alkyl group containsfrom 1 to about 8 carbon atoms. Tertiary amines are preferable to theprimary or secondary amines. A representative tertiary amine istripropylamine. Preferred, however, are the quaternary ammonium salts,which are tetraalkyl ammonium salts of strong acids, the alkyl groupcontaining from about 1 to about 8 carbon atoms. A representativequaternary ammonium salt is tetrapropylammonium (TPA) bromide.

A minor amount of alkali metal (e.g., sodium) salt may be used inaddition to the amine or quaternary ammonium salt, but the latter mustconstitute the major source of exchangeable ions in the molecular sieveas synthesized.

The amine or quarternary ammonium salt and the silica source may beadded simultaneously to the acidified iron (III) solution is desired,provided that the pH of the solution is maintained in the acidic stateand preferably at a pH not over about 5 until addition of most of thesilica source is complete. (When simultaneous addition is used, additionof the silica source may be completed before addition of the amine orquaternary ammonium salt is completed). However, it is ordinarilypreferred to add the amine or quaternary ammonium salt after all of thesilca source has been added.

The mole ratios of quaternary ammonium compound, iron compound andsilica source expressed as R₂ O, Fe₂ O₃ and SiO₂ respectively, in thereactants are substantially the same as the ratio in the as synthesizedferrisilicate gel.

Ferrisilicate gel is placed in an autoclave and heated under autogenouspressure at about 100° to about 250° C. (preferably about 170° C.) for 2to 5 days. The resulting white solid may be separated from the motherliquor, e.g., by filtration or centrifugation, then washed with waterand dried at about 100° C. The resulting material is an as synthesizedhighly crystalline ferrisilicate molecular sieve. X-ray powderdiffraction confirms the formation of the ZSM-5 structure.

The as synthesized molecular sieve is thermally treated. This generallycauses a portion of the iron to migrate from the framework to theinternal surfaces (and to a slight degree to the external surfaces aswell). Thermal treatment comprises treatment with nitrogen, air and/orsteam at a temperature from about 250° C. to about 1000° C. Preferredthermal treatment according to this invention includes treatment in aninert atmosphere, preferably a flowing stream of nitrogen, at atemperature from about 450° to about 800° C. for about 6 to about 16hours, followed by calcining in air at a temperature from about 400° toabout 1000° C. for about 3 to about 8 hours. The extent of ironmigration depends on the treating agent or agents used (steam causingthe greatest migration), and the temperature and time of treatment.Thermal treatment causes decomposition of the organic material (amine orquaternary ammonium salt). At least a portion of the thermal treatmentshould be with air in order to assure complete decomposition.

After calcination with nitrogen and air, or with air alone, theferrisilicate molecular sieve is ammonium ion exchanged, in order toremove any sodium ion present. This may be done with an aqueous solutionof an ammonium salt of strong mineral acid, such as ammonium nitrate.

After ammonium ion exchange, the molecular sieve is again thermallytreated, either by calcination in air or by hydrothermal treatment withsteam. According to one mode of treatment, the molecular sieve may beair dried at about 100°-120° C., then heated at a somewhat highertemperature, (e.g. about 250° to about 350° C.) for about 2 to 6 hours,and then calcined at high temperature, (e.g. about 550° to about 650°C.) for a longer time, (e.g. 6 to 24 hours). Finally, the calcinedmaterial may be ion exchanged, for example with potassium ion (as diluteKOH to a pH of 8.0), washed with water, filtered and air dried at about100°-120° C. The product molecular sieve formed in this manner typicallycontains about 50-100 percent of the iron in the framework, theremainder (about 0 to 50 percent) being finely dispersed throughout themolecular sieve, including the pore surfaces. Only a small amount of thenon-framework iron is on the outside surfaces, which is desirablebecause iron on the outside surfaces is less reactive for catalyticreaction purposes. The non-framework iron is in the form of particles ofiron oxides; nearly all the particles on internal surfaces are smallerthan 0.5 A while those on external surfaces are predominantly from 5 Ato 15 A.

The ammonium exchanged molecular sieve described above may behydrothermally treated with steam at about 300° to about 700° C. forabout 1 to 4 hours, washed with water, filtered and air dried at about100° to 120° C. Hydrothermal treatment with steam causes a much largerportion of the framework iron to migrate outside the framework and tobecome dispersed as finely divided iron oxide particles on the poresurfaces. Hydrothermal treatment with steam also causes a greaterpercentage of the non-framework iron to migrate to the external surfacesthan is the case when a molecular sieve is thermally treated withnitrogen and air only, or with air alone. For example, a molecular sievetreated with steam at 550° to 650° C. for 1 to 4 hours may contain about15 to 40 percent of the iron in the framework, and conversely about 60to about 85 percent of the iron outside the framework, principally inthe form of finely divided iron oxide particles not larger than about 5Angstroms dispersed mainly on the internal surfaces. Typically about95-97 percent of total non-framework iron in thermally treated molecularsieves (less in those having a SiO² /Fe2O³ mole ratio less than 50) ison the internal surfaces.

The ion exchange capacity of final product molecular sieves may bedetermined by ion exchange with dilute KOH to pH 8.0 prior to finalwashing and drying if desired.

The unit cell diameter of molecular sieves of this invention ranges fromabout 5330 A (at a SiO₂ /Fe₂ O₃ mole ratio of 100) to about 5410 A (at aSiO₂ /Fe₂ O₃ mole ratio of 20). Little further change in the unit celldiameter takes place as the SiO₂ /Fe₂ O₃ ratio is increased above 100.

When molecular sieves according to this invention are used for catalyticpurposes, the materials should generally be available in the form ofparticles with a diameter of about 0.5 to about 5 millimeters. Typicallythe final product molecular sieve have a particle diameter in the rangeof about 0.5 to about 8 microns. To achieve larger size, and to increasethermal stability, the molecular sieve may be composited with aninorganic matrix or binder material if desired. Examples of suitablematrix or binder materials are naturally occuring clays, such as kaolinand bentonite. Other suitable matrix or binder materials are syntheticinorganic oxide, such as alumina, silica, zirconia or combinationsthereof, as for example silica-alumina and silica-zirconia. The ratio ofmolecular sieve to matrix material may be as desired, and typicallymolecular sieve constitutes from about 10 to about 100 percent by weightof a composite.

Molecular sieves according to this invention may be used as catalysts invarious reactions, but are particularly suitable as Fischer-Tropschcatalysts for the direct conversion of mixtures of carbon monoxide andhydrogen to hydrocarbons, without forming and recovering methanol as anintermediate. The carbon monoxide-hydrogen mixture may be derived byconventional means, as for example steaming of coal. The mole ratio ofCO to H₂ in the reactant mixture may range from about 1:1 to about 3:1.Such a reactant mixture is contacted with a molecular sieve of thisinvention under reaction conditions, e.g. a pressure ranging from aboutatmospheric to about 20 atmosphers, a temperature ranging from about250° to about 400° C., and at a weight hourly space velocity from about0.1 to about 100 reciprocal hours (h⁻¹). The reaction product is ahydrocarbon mixture. The reaction mixture formed includes both thereaction product and unreacted carbon monoxide and hydrogen. The term"reaction product", in this specification is used to denote only thosematerials produced in the chemical reaction, while "reaction mixture"denotes the mixture of reaction product and unreacted starting materialsobtained).

Ferrisilicate molecular sieves of this invention may also be used forother catalytic reactions, particularly iron oxide-catalyzed catalyticreactions. In particular, the molecular sieves of this invention may beused as dehydrogenation and oxidation catalysts, e.g. in the oxidationof butene to butadiene, oxidation of olefins to alkane acetate esters,dehydrogenation of ethylbenzene to styrene, and oxydehydrogenation ofisobutyric acid to methacrylic acid or a lower alkyl ester thereof.Other reactions include decomposition of 2-butanol. Catalysts of thisinvention can also be used by hydrogenation catalysts for liquificationof coal.

A major advantage of the molecular sieves of this invention is theirability to catalyze direct formation of hydrocarbons, particularlygasoline hydrocarbons from carbon monoxide-hydrogen mixtures without thenecessity of producing and recovering methanol as an intermediate.Furthermore, the catalysts of this invention have good selectivity forthis reaction, which is believed due to the dispersion of iron oxides onthe internal surfaces (i.e. the pores) of the molecular sieve with onlya comparatively small amount of iron oxides on the external surface.(Iron oxides dispersed on external surfaces tend to catalyze reactionsnon-selectively, while iron oxides dispersed on internal surfacespromote selective reactions). Molecular sieves of this invention arealso selective catalysts for other iron oxide-catalyzed reactions.

Molecular sieves of this invention are also useful as isomerization andcracking catalysts. For example, they may be used for isomerization ofstraight chain alkanes to branch chain alkanes, e.g. n-hexane toisohexane. They are also useful as cracking catalysts for cracking heavyhydrocarbon fractions to produce gasoline range hydrocarbons. Thepreferred molecular sieves for isomerization and cracking are those inwhich a major portion, e.g. from about 50 to about 100 percent of totaliron, remains in the framework after thermal treatment.

This invention will be further described with reference to the exampleswhich follow. The SiO₂ /Fe₂ O₃ ratio given in each example refers tooverall mole ratio.

Samples of both as synthesized and thermally treated molecular sieveswere analyzed according to the procedures indicated below.

The x-ray powder defraction data were obtained using a Phillips X-RayDifractometer (Ni filtered Cu K-alpha, 2-theta range 5°-40°). Forcomparison, known samples of the aluminosilicate ZSM-5 were used.Chemical analysis of the samples was done by atomic absorptionspetroscopy. SEM analysis was conducted using a Cambridge ScanningElectron Microscope with Trace Northern X-ray detector.

Mossbauer spectra were measured using a conventional constantacceleration spectrometer, using a source of ⁵⁷ Co in Rh. Spectra wererecorded in O field at room temperature (RT) or liquid nitrogentemperature (LNT; 77° K.) and at 4.2° K. with either a low magneticfield (0.05 T) or a high field (8 T) applied parallel to the gamma raydirection. All isomer shifts are quoted relative to an absorber ofmetallic iron at room temperature. Fits at low field were performedusing a standard lease square fitting routine. When fitting quadrupledoublets, both peaks were constrained to have the same line with andintensity. Hyperfine split sextets were fit to 3 doublets and thehyperfine field estimated splitting of the outermost line. In all cases,it was found that the quadropole splitting (magnetic) was negative 0mms/second. Average isomer shifts were calculated directly from the rawdata by summation or from the fits. High field fits of paramagneticspectra were obtained using a spin Hamiltonian simulation program.

Mossbauer spectral analysis disclose: the distribution between frameworkand non-framework iron, and the approximate particle size of the latter.Framework iron is in the form of tetrahedra, which in the Mossbauerspectra are indicated by a singlet or single peak (when absorption ismeasured against velocity in mm/sec) regardless of measurementtemperature, with an average isomer shift (IS) no more than 0.3 mm/secat room temperature. A doublet in the Mossbauer spectrum indicates amixture of tetrahedral and octahedral iron, with the latter dispersed ina fine state of subdivision (no larger than 0.6 nanometers). A sexlet orsix-line spectrum indicates the presence of large agglomerates (largerthan 0.6 nanometers) of iron oxides. The Mossbauer spectrum indicatesthe presence of all iron present, regardless of its location.

Color of all samples was observed. All as synthesized samples werewhite, indicating that all or nearly all of the iron is in the frameworkat this stage. Thermally treated samples range from off-white to brownin color. Any discoloration indicates that at least part of the iron ispresent outside the framework as iron oxides. Color furnishes aqualitative indication as to the presence or absence of non-frameworkiron.

All thermally treated samples were base exchanged with the dilutepotassium hydroxide to pH 8.0 in order to obtain base exchange capacity.Both the acidity and the base exchange capacity diminish as the amountof framework iron decreases. Therefore, observed base exchange capacityfurnishes a confirmation of the amount of framework iron as determinedby atomic absorption spectroscopy.

EXAMPLE 1 As-Synthesized Ferrisilicate Molecular Sieve. SiO₂ /Fe₂ O₃ =98

100 g N-brand Silica (PQ Corp., Valley Forge, PA, Na₂ SiO₃.5H₂ O) in 100g H₂ O, is added to a solution containing 4.16 g Fe(NO₃)₃.9H₂ Odissolved in 50 g H₂ O. The pH is adjusted to be strongly acidic with7.5 g H₂ SO₄ (96 percent). To the resulting pale lemon colored gel isadded 12 g tetrapropylammonium bromide (Aldrich Chemical) in 50 g H₂ O.After vigorous agitation the mixture is placed in a stainless steelautoclave, sealed and heated under its own pressure at 170° C. for 2 to5 days, without stirring. The resulting white solid is filtered, washedwith water and dried at 100° C. X-ray powder diffraction confirms theformation of the ZSM-5 structure. The material contains almost all ofits iron in the framework of the molecular sieve, as shown by Mossbauerspectroscopy.

EXAMPLE 2 Thermally Modified Ferrisilicate Molecular Sieve. SiO₂/Fe.sub. 2 O₃ =98

The sample from Example 1 is heated in air at 110° C. for three hours.It is then carefully calcined in dry air in two stages. In the firststage it is heated in dry air at 300° C. for 2-3 hours followed by asecond stage, where it is calcined in dry air at 600° C. for 18 hours.The sample obtained was then ammonium exchanged using excess 1M ammoniumnitrate solution at 65° C. for two hours. The resulting solid sample isnamed as sample A for future reference herein. A portion of the sample Ais heated at 110° C. for two hours. It is then heated in dry air at 300°C. for three hours and calcined in dry air at 600° C. for 18 hours. Thesample is then potassium exchanged by titrating it with dilute KOH to apH of 8.0. The sample is finally washed with water, filtered and airdried at 110° C. This material contains 71 percent of the iron in theframework of the molecular sieve. The remaining 29 percent of the ironis very finely dispersed throughout the molecular sieve, including theinside of the pores.

EXAMPLE 3 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderMild Conditions. SiO₂ /Fe₂ O₃ =98

A portion of the sample A (of Example 2) is hydrothermally treated usingsteam at 650° C. for one hour. The sample is then potassium exchanged bytitrating it with dilute KOH to a pH of 8.0. The resulting sample iswashed with water, filtered and air dried at 110° C. This materialcontains 35 percent of the iron in the framework of the molecular sieve.The remaining 60 percent of the iron is outside the framework and ispresent inside and outside the pores of the molecular sieve.

EXAMPLE 4 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderSevere Conditions SiO₂ /Fe₂ O₃ =98

A portion of the sample A (of Example 2) is hydrothermally treated usingsteam at 600° C. for 4 hours. The sample is then potassium exchanged bytitrating it with dilute KOH to a pH of 8.0. The resulting sample iswashed with water, filtered and air dried at 110° C. This materialcontains only 20 percent of the total iron in the framework of themolecular sieve. The remaining 80 percent of the iron is outside theframework and is present inside and outside the pores of the molecularsieves.

EXAMPLE 5 As Synthesized Molecular Sieves; SiO₂ /Fe₂ O₃ ranging from 20to 200 (General Procedure)

The iron containing reagent (Fe(NO₃)₃. 9H₂ O, or FeCl₃.H₂ O) FisherReagent Grade) was dissolved in 100 g H₂ O. The solution, was acidifiedwith 16 g H₂ SO₄ (96 percent) and 200 g N-brand silica (PQ Corp. Na₂SiO₃.5H₂ O) in 200 g H₂ O was added to this fresh solution. Immediateformation of a pale yellow gel was observed. To the gel was added 24 gtetrapropylammonium bromide (TPABr Aldrich Reagent Grade) in 40 g H₂ O.The gel was heated in a stirred autoclave (Autoclave Engineers 1dm³capacity) at 170° C. for 3 days under autogeneous pressure. Theresulting white solid was filtered, washed and dried at 100° C. X-raypowder diffraction confirmed the presence of highly crystallineferrisilicate with the zeolite ZSM-5 structure. Atomic absorptionconfirms SlO₂ /Fe₂ O₃ ratio.

FeZSM-5 (20): To 3.4 g, FeCl₃.H₂ O in 50 gH₂ O with 4.5 g H₂ SO₄ isadded 50 g N-brand silica in 50 g H₂ O. to the resulting gel was added6.3 g TPABr in 10 g H₂ O.

FeZSM-5 (50): To 15 g Fe(NO₃)₃.9H₂ O in 100 g H₂ O and 16 g H₂ SO₄ wasadded 200 g N-brand silica in 200 g H₂ O. After precipitates of the gel,24 g TPABr in 40 g H₂ O was added.

FeZSM-5 (90): To 4.16 g Fe (NO₃)₃.9H₂ O dissolved in 50 g H₂ O andacidified with 7.5 g H₂ SO₄ was added 100 g N-brand silicate in 100 g H₂O. After formation of the milky gel, 12 g TPABr in 25 g H₂ O was added.

FeZSM-5 (171): to 3.5 g Fe(NO₃)₃.9H₂ O dissolved in 100 g H₂ O and 16 gH₂ O₄ was added 200 g N-brand silica in 200 g H₂ O. After the whitemilky gel formed, 24 g TPABr in 50 g H₂ O was added.

Silicalite: To a 15 g H₂ SO₄ in 75 g H₂ O solution was added 150 gN-brand silicate in 150 g H₂ O to this was added 18 g TPA Br in 100 g H₂O. (This is included for comparison).

EXAMPLE 6 Thermally Modified Ferrisilicate Molecular Sieve; SiO₂ /Fe₂ O₃=20

The as-synthesized sample of ferrisilicate molecular sieve with SiO₂/Fe₂ O₃ of 20 is dried in air at 110° C. for three hours. It is thenheated in dry nitrogen for two hours at 145° C., followed by heating at500° C. for 8-10 hours. It is cooled to 145° C. in dry nitrogen and thenswitched to dry air at 145° C. for 2 hours. Finally, the sample iscalcined in dry air at 500° C. for 4 to 5 hours.

An ammonium exchanged form of the sample was obtained by ammoniumexchange using excess 1M ammonium nitrate solution at 65° C. for twohours.

A portion of this sample is heated at 110° C. for two hours, then heatedin dry air at 145° C. for two hours and finally calcined in dry air at500° C. for 5 hours. The sample is then potassium exchanged by titratingit with dilute KOH to a pH of 8.0.

EXAMPLE 7 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderVery Mild Conditions SiO₂ /Fe₂ O₃ =20

A portion of the ammonium exchanged sample (of Example 6) ishydrothermally treated using steam at 300° C. for one to four hours. Thesample is then potassium exchanged by titrating it with dilute KOH to apH of 8.0. The resulting sample is washed with water, filtered and airdried at 110° C.

EXAMPLE 8 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderMild Conditions. SiO₂ /Fe₂ O₃ =20

A portion of the ammonium exchanged sample (of Example 6) ishydrothermally treated using steam at 550° C. for one hour. The sampleis then potassium exchanged by titrating it with dilute KOH to a pH of8.0. The resulting sample is washed with water, filtered and air driedat 110° C.

EXAMPLE 9 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderModerate Conditions; SiO₂ /Fe₂ O₃ =20

Two portions of the ammonium exchanged samples (of Example 6)hydrothermally treated using steam at 550° C. for two and four hoursrespectively. The two portions are then potassium exchanged separatelyby titrating them with dilute KOH to a pH of 8.0. The resulting samplesare washed with water, filtered and air dried at 110° C.

EXAMPLE 10 Hydrothermally Modified Ferrisilicate Molecular Sieves UnderModerate Conditions for Long Periods of Time; SiO₂ /Fe₂ O₃ =20

Portions of the ammonium exchanged samples of Example 6 arehydrothermally treated using steam at 550° C. for 8 to 72 hours. Thesamples are then potassium exchanged by titrating them separately withdilute KOH to a pH of 8.0. The resulting samples are washed with water,filtered and air dried at 110° C.

EXAMPLE 11 Hydrothermally Modified Ferrisilicate Molecular Sieves UnderSevere Conditions; SiO₂ /Fe₂ O₃ =20

Portions of the ammonium exchanged samples of Example 6 arehydrothermally treated using steam at 600° to 700° C. for one to 8hours. The samples are then potassium exchanged by titrating themseparately with dilute KOH to a pH of 8.0. The resulting samples arewashed with water, filtered and air dried at 110° C.

EXAMPLE 12 Thermally Modified Ferrisilicate Molecular Sieve; SiO₂ /Fe₂O₃ =50

The as synthesized sample of ferrisilicate molecular sieve with SiO₂/Fe₂ O₃ of 50 is heated in air at 110° C. for three hours. It is thenheated in dry nitrogen for two hours at 145° C. followed by heating at500° C. in dry nitrogen for 8-10 hours. It is cooled to 145° C. in drynitrogen and then switched to dry air at 145° C. for 2 hours. Finally,the sample is calcined in dry air at 500° C. for 4 to 5 hours.

An ammonium exchanged form of the sample was obtained by ammoniumexchange using excess 1M ammonium nitrate solution at 65° C. for 2hours.

For hydrothermal modification of the material, an ammonium exchangedform of the sample is otained by ammonium exchange using excess 1Mammonium nitrate solution at 65° C. for two hours.

A portion of this sample is heated at 110° C. for two hours, then heatedin dry air at 145° C. for two hours and finally calcined in dry air at500° C. for 5 hours. The sample is then potassium exchanged by titratingit with dilute KOH to a pH of 8.0.

EXAMPLE 13 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderVery Mild Conditions; SiO₂ /Fe₂ O₃ =50

A portion of the ammonium exchanged sample of Example 12 ishydrothermally treated using steam at 300° C. for one to four hours. Thesample is then potassium exchanged by titrating it with dilute KOH to apH of 8.0. The resulting sample is washed with water, filtered and airdried at 110° C.

EXAMPLE 14 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderMild Conditions; SiO₂ /Fe₂ O₃ =50

A portion of the ammonium exchanged sample of Example 12 ishydrothermally treated using steam at 550° C. for one hour. The sampleis then potassium exchanged by titrating it with dilute KOH to a pH of8.0. The resulting sample is washed with water, filtered and air driedat 110° C.

EXAMPLE 15 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderModerate Conditions; SiO₂ /Fe₂ O₃ =50

Two portions of the ammonium exchanged sample of Example 12 ishydrothermally treated using steam at 550° C. for two and four hoursrespectively. The two portions are potassium exchanged separately bytitrating them with dilute KOH to a pH of 8.0. The resulting samples arewashed with water, filtered and air dried at 110° C.

EXAMPLE 16 Hydrothermally Modified Ferrisilicate Molecular Sieves UnderModerate Conditions for Long Periods of Time; SiO₂ /Fe₂ O3=50

Portions of the ammonium exchanged samples of Example 12 arehydrothermally treated using steam at 550° C. for 8 to 72 hours. Thesamples are then potassium exchanged by titrating them separately withdilute KOH to a pH of 8.0. The resulting samples are washed with water,filtered and air dried at 110° C.

EXAMPLE 17 Hydrothermally Modified Ferrisilicate Molecular Sieves UnderSevere Conditions; SiO₂ /Fe₂ O₃ =50

Portions of the ammonium exchanged samples of Example 12 arehydrothermally treated using steam at 600° to 700° C. for one to 8hours. The samples are then potassium exchanged by titrating themseparately with dilute KOH to a pH of 8.0. The resulting samples arewashed with water, filtered and air dried at 110° C.

EXAMPLE 18 Thermally Modified Ferrisilicate Molecular Sieve; SiO₂ /Fe₂O₃ =90

The as-synthesized sample of ferrisilicate molecular sieve with SiO₂/Fe₂ O₃ of 90 is heated in air at 110° C. for three hours. It is thenheated in dry nitrogen for two hours at 145° C. followed by heating itat 500° C. in dry nitrogen for 8-10 hours. It is cooled to 145° C. indry nitrogen and then switched to dry air at 145° C. for 2 hours.Finally, the sample is calcined in dry air at 500° C. for 4 to 5 hours.

For hydrothermal modification of the material, an ammonium exchangedform of the sample is obtained by ammonium exchange using excess 1Mammonium nitrate solution at 65° C. for two hours.

A portion of this sample is heated at 110° C. for two hours, then heatedin dry air at 145° C. for two hours and finally calcined in dry air at500° C. for 5 hours. The sample is then potassium exchanged by titratingit with dilute KOH to a pH of 8.0.

EXAMPLE 19 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderVery Mild Conditions; SiO₂ /Fe₂ O₃ =90

A portion of the ammonium exchanged sample of Example 18 ishydrothermally treated using steam at 300° C. for one to four hours. Thesample is then potassium exchanged by titrating it with dilute KOH to apH of 8.0. The resulting sample is washed with water, filtered and airdried at 110° C.

EXAMPLE 20 Hydrothermally Modified Ferrisilicate Molecular Sieve undermild Conditions; SiO₂ /Fe₂ O₃

A portion of the ammonium exchanged sample of Example 18 ishydrothermally treated using steam at 550° C. for one hour. The sampleis then potassium exchanged by titrating it with dilute KOH to a pH of8.0. The resulting sample is washed with water, filtered and air driedat 110° C.

EXAMPLE 21 Hydrothermally Modified Ferrisilicate Molecular Sieve undermoderate Conditions; SiO₂ /Fe₂ O₃ =90

Two portions of the ammonium exchanged sample of Example 18 arehydrothermally treated using steam at 550° C. for two and four hoursrespectively. The two portions are potassium exchanged separately bytitrating them with dilute KOH to a pH of 8.0. The resulting samples arewashed with water, filtered and air dried at 110° C.

EXAMPLE 22 Hydrothermally Modified Ferrisilicate Molecular Sieves undermoderate Conditions for long Periods of Time; SiO₂ /Fe₂ O₃ =90

Portions of the ammonium exchanges samples of Example 18 arehydrothermally treated using steam at 550° C. for 8 to 72 hours. Thesamples are then potassium exchanged by titrating them separately withdilute KOH to a pH of 8.0. The resulting samples are washed with water,filtered and air dried at 110° C.

EXAMPLE 23 Hydrothermally Modified Ferrisilicate Molecular Sieves undersevere Conditions SiO₂ /Fe₂ O₃ =90

Portions of the ammonium exchanged samples of Example 18 arehydrothermally treated using steam at 600 ° to 700° C. for one to 8hours. The samples are then potassium exchanged by titrating them seithdilute KOH to a pH of 8.0. The resulting samples are washed with water,filtered and air dried at 110° C.

EXAMPLE 24 Thermally Modified Ferrisilicate Molecular Sieve; SiO₂ /Fe₂O₃ =200

The as-synthesized sample of ferrisilicate molecular sieve with SiO₂/Fe₂ O₃ of 200 is heated in air at 110° C. for three hours. It is thenheated in dry nitrogen for two hours at 145° C. followed by heating itat 500° C. in dry nitrogen for 8 to 10 hours. It is cooled to 145° C. indry nitrogen and then switched to dry air at 145° C. for 2 hours.Finally, the sample is calcined in dry air at 500° C. for 4 to 5 hours.

For hydrothermal modification of the material, an ammonium exchangedform of the sample was obtained by ammonium exchange using excess 1Mammonium nitrate solution at 65° C. for two hours.

A portion of this sample is heated at 110° C. for two hours, then heatedin dry air at 145° C. for two hours and finally calcined in dry air at500° C. for 5 hours. The sample is then potassium exchanged by titratingit with dilute KOH to a pH of 8.0.

EXAMPLE 25 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderVery Mild Conditions; SiO/Fe₂ O₃ =200

A portion of the ammonium exchanged sample of Example 24 ishydrothermally treated using steam at 300° C. for one to four hours. Thesample is then potassium exchanged by titrating it with dilute KOH to apH of 8.0. The resulting sample is washed with water, filtered and airdried at 110° C.

EXAMPLE 26 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderMild Conditions; SiO₂ /Fe₂ O₃ =200

A portion of the ammonium exchanged sample of Example 24 ishydrothermally treated using steam at 550° C. for one hour. The sampleis then potassium exchanged by titrating it with dilute KOH to a pH of8.0. The resulting sample is washed with water, filtered and air driedat 110° C.

EXAMPLE 27 Hydrothermally Modified Ferrisilicate Molecular Sieve UnderModerate Conditions SiO₂ /Fe₂ O₃ =200

Two portions of the ammonium exchanged sample of Example 24 arehydrothermally treated using steam at 550° C. for two and four hoursrespectively. The two portions are then potassium exchanged, separatelyby titrating them with dilute KOH to a pH of 8.0. The resulting samplesare washed with water, filtered and air dried at 110° C.

EXAMPLE 28 Hydrothermally Modified Ferrisilicate Molecular Sieves UnderModerate Conditions for Long Periods of Time; SiO₂ /Fe₂ O₃ =200

Portions of the ammonium exchanged samples of Example 24 arehydrothermally treated using steam at 550° C. for 8 to 72 hours. Thesamples are then potassium exchanged by titrating them separately withdilute KOH to a pH of 8.0. The resulting samples are washed with water,filtered and air dried at 110° C.

EXAMPLE 29 Hydrothermally Modified Ferrisilicate Molecular Sieves UnderSevere Conditions; SiO₂ /Fe₂ O₃ =200

Portions of the ammonium exchanged samples of Example 24 arehydrothermally treated using steam at 600° to 700° C. for one to 8hours. The samples are then potassium exchanged by titrating themseparately with dilute KOH to a pH of 8.0. The resulting samples arewashed with water, filtered and air dried at 110° C.

EXAMPLE 30 Preparation of Gasoline Range Hydrocarbons From Synthesis Gas

Mixtures of carbonmonoxide and hydrogen (synthesis gas) are passed overthe thermally treated ferrisilicate molecular sieve of Example 12 underthe following process conditions: H₂ /CO ratio of 1.0 to 3.0, pressureof 15 to 30 atmospheres, temperature of 300° C. to 400°, flow rates of 8to 60 cc/min. The weight of catalyst is approximately 0.4 grams. Theproducts include gasoline range hydrocarbons.

EXAMPLE 31 Preparation of Light Hydrocarbons From Synthesis Gas

A series of experiments was conducted in a plug-flow micro-reactorsystem using a hydrothermally treated (for 2 hours at 500° C.) molecularsieve having a SiO₂ /Fe₂ O₃ ratio of 50, prepared according to Example15. The H₂ /CO ratio in these experiments is set at 3. Approximately 0.4gram of catalyst is used in each run. Two series of experiments, one atone atmosphere, the other at 12 atmospheres, are carried out. Flow ratesand other process conditions are varied as shown in Table II below. Alsoshown in Table II are the product distributions attained in each run.

                  TABLE II                                                        ______________________________________                                        Run      1       2       3     4     5     6                                  ______________________________________                                        Pressure,                                                                                1.0     1.0     1.0 12    12    12                                 atm                                                                           Reactor  250     300     350   250   300   350                                temp. °C.                                                              Feed gas   60.0    60.0    60.0                                                                              25    8     6                                  flow rate,                                                                    cc/.min                                                                       Product distribution, mole %                                                  Methane  52      45      45    50    45    47                                 Ethane   --       8       8     2    22    20                                 Ethylene 25      24      24    12    1     1                                  Propane  --      --       2     6    12    14                                 Propylene                                                                              23      21      19     9    2     1                                  C.sub.4  --      --      --    --    7     8                                  C.sub.5  --      --      --    --    4     4                                  C.sub.6  --      --      --    --    3     2                                  C.sub.7  --      --      --    --    1     1                                  C.sub.8  --      --      --    --    1     trace                              ______________________________________                                    

All percentages of hydrocarbon products in the above Table II are basedon the amount of CO converted.

While in accordance with patent statutes, a preferred embodiment andbest mode has been presented, the scope of the invention is not limitedthereto, but rather is measured by the scope of the attached claims.

What is claimed is:
 1. A thermally treated crystalline ferrisilicatemolecular sieve having the structure of ZSM-5, said ferrisilicatemolecular sieve having an overall SiO₂ /Fe₂ O₃ mole ratio in the rangeof about 20 to about 400, about 15 to about 40 percent of the ironcontent being in the crystal framework and the remaining portion beingoutside the crystal framework, said remaining portion constituting fromabout 60 to about 85 percent by weight of the total iron content andbeing dispersed in the form of finely divided particles on the internaland external surfaces of the molecular sieve, at least about 30 percentof non-framework iron being dispersed on the internal surfaces, saidmolecular sieve being prepared by a process which comprises:(a) adding asilica source and one or more compounds selected from the groupconsisting of primary, secondary and tertiary amines, and quaternaryammonium compounds to an acidic aqueous solution of an iron (III)compound, and maintaining said solution in the acidic state until theaddition of said silica source is complete; (b) heating the mixtureobtained in step (a) at a temperature of about 100° C. to about 250° C.until molecular sieve crystals are formed; and (c) thermally treatingthe molecular sieve crystals formed in step (b) in an inert atmosphereat about 400° C. to about 1000° C. for about 3 to about 8 hours, andthen with steam at a temperature from about 300° C. to about 700° C. forabout 1 to about 4 hours."
 2. A crystalline ferrisilicate molecularsieve according to claim 1 in which at least about 50 percent ofnon-framework iron is dispersed on the internal surfaces.
 3. Acrystalline ferrisilicate molecular sieve according to claim 1 in whichthe non-framework iron consists essentially of iron oxide particles lessthan 5 Angstrom units in size.
 4. A crystalline ferrisilicate molecularsieve according to claim 1 in which the overall SiO₂ /Fe₂ O₃ mole ratiois from about 30 to about
 200. 5. A synthetic ferrisilicate molecularsieve according to claim 1 in which at least about 80 percent of thenon-framework iron is dispersed on internal surfaces.